The invention has particular utility in providing a relatively simple means which can be used for measuring the scan profile of a rotating mirror and is adapted to provide a precise measurement of position versus time without mechanically affecting the performance of the mirror. Accordingly, the invention has widespread application wherever it is desired to determine angular velocity or angular displacement of a moving object without affecting the motion of the object and without having the measurement influenced by linear displacements of the object.
In the past, measurement of the rotation of an object, such as a mirror, was made by placing a precisely divided graticule in the focal plane of a lens. A collimator beam was then reflected off the rotating mirror and the position of the reflected beam in the lens focal plane with respect to the graticule was read to determine the rotation angle. The disadvantage of this method is that the distortions in the lens must not only be known, but the lens itself must have a relatively wide stigmatic field and the graticule must be relatively large and precisely measured.